delete properties in messed up file
You can delete that property, the error you get is probably (here recreated):

&gt;&gt;&gt; bpy.types.Scene.ragamuffin = bpy.props.IntProperty(default=1)

&gt;&gt;&gt; bpy.types.Scene.ragamuffin
(&lt;built-in function IntProperty&gt;, {'default': 1, 'attr': 'ragamuffin'})

&gt;&gt;&gt; bpy.data.scenes['Scene'].ragamuffin
1

&gt;&gt;&gt; del bpy.data.scenes['Scene'].ragamuffin
Traceback (most recent call last):
  File "&lt;blender_console&gt;", line 1, in &lt;module&gt;
AttributeError: bpy_struct: del not supported


But there's no guarantee that will make the scene work again..
removal anyway:

&gt;&gt;&gt; del bpy.types.Scene.ragamuffin    # success!
&gt;&gt;&gt; bpy.types.Scene.ragamuffin
Traceback (most recent call last):
  File "&lt;blender_console&gt;", line 1, in &lt;module&gt;
AttributeError: type object 'Scene' has no attribute 'ragamuffin'


I can't for certain say what produced those properties, but it's worth having a look at the source of the plugin. For instance here RIBMosaic implements some unusual (not bad...just not standard bpy style) property registration techniques using exec. meaning you can't easily tell in advance exactly what the properties will be called.